Project Summary Inflammatory bowel diseases (IBD), comprised of Crohn?s disease (CD) and ulcerative colitis (UC), afflict approximately one million Americans and millions more worldwide. Alarmingly, the overall incidence of IBD is increasing, and increasing in children and persons in whom IBD had previously been uncommon. In addition to significant impairment from intestinal inflammation, affected patients suffer from extra-intestinal manifestations of disease, morbidities from medical and surgical treatment, and increased risk of colon cancer. Treatments are costly and ineffective in many patients. The mechanisms of IBD are complex and therefore not well understood. The importance of T cells in mediating aberrant adaptive immune responses in IBD is well established, however, the mechanisms through which T cells mediate disease remain unclear. While it is likely that CD4+ T cells guide and regulate CD8+ T cell responses, the end effectors that drive tissue damage are likely cytotoxic CD8+ T cells. Our proposal specifically aims to identify and study effector CD8+ T cells in UC- the cells we propose are responsible for tissue damage. Our team recently developed novel tools that enable us to glean unprecedented amounts of information from single T cells, accurately identify their TCR sequence, and query their antigen specificity. Our central hypothesis is that Bcl6-expressing CD8+ T cells drive tissue damage in UC. Further, we hypothesize that they are responding to locally derived signals. If so, we reason that these locally derived signals can be pharmacologically targeted. We will investigate these hypotheses through the following specific aims: 1) Investigate mechanisms of intestinal damage in ulcerative colitis through the single-cell study and functional characterization of bcl6-expressing CD8+ T cells. 2) Investigate antigen specificity of bcl6-expressing CD8+ T cells through intestinal organoids and random peptide screening. Successful completion of these aims could have important implications in therapies. The proposed research is innovative because it applies innovative methods to study T cell function within UC through single-cell analysis of TCR-matched cells from inflamed vs. non-inflamed colon within the same patients. Our own expertise and the expertise of our collaborators make us uniquely qualified to perform these studies.